Microelectromechanical systems (MEMS), for example, gyroscopes, resonators and accelerometers, utilize micromachining techniques (i.e., lithographic and other precision fabrication techniques) to reduce mechanical components to a scale that is generally comparable to microelectronics. MEMS typically include a mechanical structure fabricated from or on, for example, a silicon substrate using micromachining techniques.
The mechanical structures in MEMS devices are typically sealed in a chamber. The delicate mechanical structure may be sealed in, for example, a hermetically sealed metal container (for example, a TO-8 “can” as described in U.S. Pat. No. 6,307,815) or bonded to a semiconductor or glass-like substrate having a chamber to house, accommodate or cover the mechanical structure (see, for example, U.S. Pat. Nos. 6,146,917; 6,352,935; 6,477,901; and 6,507,082). In the context of the hermetically sealed metal container, the substrate on, or in which, the mechanical structure resides may be disposed in and affixed to the metal container. The hermetically sealed metal container also serves as a primary package as well.
In the context of the semiconductor or glass-like substrate packaging technique, the substrate of the mechanical structure may be bonded to another substrate whereby the bonded substrates form a chamber within which the mechanical structure resides. In this way, the operating environment of the mechanical structure may be controlled and the structure itself protected from, for example, inadvertent contact. The two bonded substrates may or may not be the primary package for the MEMS as well.
Another technique for forming the chamber that protects the delicate mechanical structure of a MEMS device employs micromachining techniques. (See, for example, International Published Patent Applications Nos. WO 01/77008 A1 and WO 01/77009 A1). In this regard, the mechanical structure is encapsulated in a chamber using a conventional oxide (SiO2) deposited or formed using conventional techniques (i.e., oxidation using low temperature techniques (LTO), tetraethoxysilane (TEOS) or the like). (See, for example, WO 01/77008 A1, FIGS. 2-4). When implementing this technique, the mechanical structure is encapsulated prior to packaging and/or integration with integrated circuitry.
When the chamber in which the mechanical structure is housed is sealed, the final pressure and the gaseous environment of the chamber are determined by the temperature, pressure, and atmosphere at the time the chamber is sealed. Accordingly, when using processes, such as a seal glass bonding process, wherein all of the chambers on a wafer are exposed to the same environment at the time the chambers area sealed, each of the chambers on the wafer have the same final pressure.
What is needed is a method of forming wafers such that different final pressures are realized in different chambers on the wafer. A further need exists for such a method which does not significantly increase the cost of producing the wafer.